People don't put on enough sunscreen to get the SPF level of protection stated on the bottle, say New Zealand and English researchers. They suggest a new way of measuring the amount of sunscreen to apply.

In a letter to this week's British Medical Journal (BMJ), Dr Steve Taylor of Sunset Road Family Doctors, Mairangi Bay, Auckland, New Zealand and Professor Brian Diffey of the Regional Medical Physics Department, Newcastle General Hospital, UK describe how most people use less than half the required amount of sunscreen for full protection.

"We know people underdo their sunscreen application. If people don't use 2 mg per square centimetre then they are not protected."

Studies have shown that consumers apply much less sunscreen than is required to achieve the sun protection factor (SPF) stated on the product. In this week's BMJ, researchers suggest a simple guide to ensure that people are protected according to their expectations.

The SPF is determined by phototesting at an internationally agreed application thickness of 2mg/cm squared. Consumers typically apply much less than this - between 0.5 and 1.5mg/cm squared, according to studies.

As a way of determining how much sunscreen to use the so-called 'rule of nines' can be invoked. This divides the body's surface area into 11 imaginary areas, each representing roughly nine per cent of the total. Head, neck and face, make up one section. The arms are another two, the upper and lower front and back make up four areas, with the remaining four areas being each of the upper legs and the lower legs and feet.

To achieve the recommended thickness, each of these body areas must receive 'two fingers' of sunscreen  that is, two strips of sunscreen squeezed out onto both the index and middle fingers, right from the fingertips to the crease where the finger meets the palm.

The application of this 'two fingers' of sunscreen will provide a dose of the product that approximates the dose used during the laboratory determination of the SPF.

In reality, users are unlikely to be willing to cover themselves or their families with such a copious layer of sunscreen and would prefer to apply half this amount, say the authors.

They therefore suggest applying one finger of sunscreen, with the corollary that the resultant protection would only be about half that stated on the product. Users should then be encouraged to reapply one finger's worth within half an hour of the initial application in order to achieve optimum protection, they conclude.

Despite the daunting prospect of smearing huge quantities of cream onto your skin, the amounts are cosmetically quite good, said Dr Menzies - meaning they will spread out rather than form a 'zinc layer' on the skin.

Formulae are very different from one sunscreen brand to another. People should try different brands until they find one that suits them. "Australian sunscreens are more acceptable than overseas competitors," he said.

Legislation in Australia states that sunscreen cannot be marketed at an SPF higher than 30. "Some sunscreens are up to an SPF of 100 but they can't market them as such here," he said.

"The message has to be that if you want to get the SPF that these sunscreens are tested at then you have to use this much," said Dr Menzies.

If people are applying less than half the amount they won't be protected. "If it is SPF 15 sunscreen then they'll only have protection of SPF 7 which means they will burn," he noted. "It is not as critical with higher SPF sunscreens."